Die Bonding is the process of attaching the semiconductor die either to its package, the metallic lead frame or to some substrate. The process starts with selecting the target die from wafer or waffle tray, aligning the selected die to a target pad on the carrier or substrate, and then permanently attaching the selected die to the target pad using one of several die bonding techniques.
Epoxy bonding is one form of die bonding technique. Here, an epoxy bond is formed by attaching the die to the substrate (e.g., leadframe die pad) with the use of epoxy glue. A drop of epoxy is dispensed on the die pad and the die placed on top of it. This process uses adhesives such as polyimide, epoxy and silver-filled glass as die attach material to mount the die on the die pad of a package. Epoxy adhesives are typically electrical insulators and have poor thermal conductivity. To improve the electrical conductivity, epoxy or polyimides are filled with metal material (e.g., gold or silver). Thus, adhesive die attach materials may be regarded as suspensions of metal particles in a carrier (i.e., conductive die bond glue). The conductive die bond glue provides adhesion and cohesion to make a bond with the correct mechanical strength, while the metal particles provide electrical and thermal conductivity. It is noticeable that conductive resins are now often used where no electrical connection is required, just to get the benefit of enhanced thermal performance.
Package delamination forms a separation layer in between mold compound to chip, die paddle and leads, which may subsequently affect ground bond quality and may degrade package electrical performance, if it goes along with other influences such as high humidity, extreme temperatures or pollution of the environment (e.g., with salt). Delamination, or the separation between two supposedly connected layer interfaces within a chip package, is generally considered more as a failure attribute rather than as a failure mechanism, i.e., its presence in a package does not necessarily mean a failure. Its presence, however, has to be considered a valid package failure if its size, location, shape, or any other characteristic poses a reliability risk in the field, i.e., it can cause the device to fail by a secondary failure mechanism.
Secondary failure mechanisms that arise from the presence of delamination include die corrosion, package cracking, bond lifting, and breaking of the neck or heel of a bond. Device-related failures such as parametric shifts due to internal contamination can also be induced by package delamination.
Delamination is often addressed in the context of which layer interfaces are involved. As such, die-to-mold delamination is often treated differently from leadframe-to-mold delamination, since they result in different failure mechanisms and require different corrective actions for elimination.
For instance, a die-to-mold delamination can cause the molding compound to move laterally with respect to the die surface, which can either cause ball bond lifting or neck breaks. If a moisture path between the die-to-mold delamination and an external feature of the package exists, then moisture and contaminants can reach the die surface from outside, resulting in die corrosion or metal-to-metal leakage.
Additionally, if moisture enters the package, the moisture may resolve metal ions from the conductive die bond glue, resulting in charge carriers in the water that collects inside the package. The metal ions may reach the bond pads of a chip due to an attraction of the ions by an electric field generated by the bond pads and may cause a leakage current to flow between output pads at different electrical potentials.
In cases of a structure with a driven interface these leakages do usually not cause an effect on the measurement, since they only represent an additional load current for the driver. However, in the case of passive elements (i.e., components without drivers), like resistive sensor bridges with a relatively high impedance, currents between the output pads can change the measurement value by an amount Rbridge*Ileakage without any chance of a detection, where Rbridge is the resistance value of the resistive sensor bridge and Ileakage is the leakage current.
Therefore, an improved device capable of detecting a presence of a leakage current (i.e., a detectable fault) and/or measuring the leakage current may be desirable.